a) Field of the Invention
The invention relates to new lithium insertion electrode materials based on orthosilicate derivatives, electrochemical generators and variable optical transmission devices having an electrode containing these materials.
b) Description of the Prior Art
Electrode materials are mainly derivatives of transition metals chalcogenides, especially oxides having a lamellar structure like Li.sub.x CoO.sub.2 or spinels like Li.sub.2x Mn.sub.2 O.sub.4. These materials usually operate via a lithium insertion mechanism with less than one lithium and electron exchanged per transition metal in the unit formula (0.ltoreq.x.ltoreq.0.6). Besides, these materials suffer from facile loss of the transition metal which is leached into the electrolyte. Cobalt is prohibitively expensive for large scale application.
Materials with the olivine structure LiFePO.sub.4 (triphylite) and the quasi-isomorphous delithiated material noted as ".quadrature.FePO.sub.4 " where ".quadrature." represents the lithium site vacancies, have the advantage of an operating voltage of 3.5 V vs. Li.sup.+ /Li.sup.o, i.e. in the stability window of both liquid and polymer electrolytes with a flat discharge (lithium intercalation) plateau. The materials are however limited by relatively both slow Li.sup.+ diffusion kinetics and low electronic conductivity due to the high electronegativity of the phosphate anion. The absence of non-stoichiometry or mutual miscibility for both phases (LiFePO.sub.4 and .quadrature.FePO.sub.4) is an advantage in terms of the shape of the discharge curve but limits the kinetics of phase transformation which may lead ultimately at high current densities to fragmentation of the material and loss of contact with the carbon grains acting as current collector. The phosphate of this type offer the possibility of exchanging only one lithium per unit formula.
Conversely, it is known that the compounds with either the Li.sub.2 SO.sub.4, Li.sub.3 PO.sub.4, Li.sub.4 SiO.sub.4 and their solid solutions possess relatively high diffusion coefficients for lithium ions, but have no redox activity.